The increasing demands for transmitting massive data requires ever increasing signal transmission speeds between components on circuit boards. To achieve these speeds, frequency ranges are necessarily increasing from below 1 MHz to, 1 GHz, 10 GHz or even higher. In these higher ranges, the currents flow mostly near the surface of the conductor due to the well-known “skin effect” which is the tendency of high frequency current density to be highest at the surface of a conductor and to decay exponentially towards the center. The skin depth, where approximately 67% of the signal is carried, is inversely proportional to the square root of the frequency. Accordingly, at 1 MHz the skin depth is 65.2 μm, at 1 GHz it is 2.1 μm, while at 10 GHz the skin depth is only 0.7 μm. At the higher frequencies, the surface topography or roughness of the conductor becomes ever more important since a roughness on the order of, or greater than, the skin depth will impact the signal transmission through scattering.
An exacerbating factor is that usually the surface of the conductor in printed circuit boards is intentionally roughened to enhance adhesion characteristics to the resin layer used in the laminated structures of circuit boards. A surface roughness, Rz, on the roughened surface on the order of several m is typical and will impact any transmission in the GHz range. The design is therefore constrained by the conflicting need for high roughness to ensure enough adhesion, and low roughness to minimize transmission loss.
One approach to try and provide control of the surface topography is to roughen either the deposited side or the drum side of an electrodeposited copper foil. The deposited side is typically rougher than the drum or “shiny” side. In a normal treated foil, the deposited side is roughened and adhesion to the resin layer is superior since the roughness is typically higher. In order to maintain the quality of signal transmission, reverse treated foil (RTF) has been developed. RTF is roughened at its shiny side, so that roughness is higher than without any roughening but can be controlled to be lower than where roughening is provided on the deposited side. Therefore, RTF provides good adhesion, for example comparable to normal treated foil or at least acceptable for applications such as for use in circuit boards. RTF also ideally has reduced transmission loss of signal when compared to normal treated foil.
Although RTF technologies can provide an improvement with regards to transmission loss, it is not always clear what surface properties of a copper foil provide an acceptable improvement. Although Rz is one parameter that can be used, in some cases this property is not predictive enough. There therefore remains a need for copper foils with low transmission loss for the manufacturing of circuit boards and copper foils with controlled properties to achieve the desired low transmission loss.